Geometric isotope effects in crystalline sodium hydrogen bis(4-nitrophenoxide) dihydrate

The crystal structure of sodium hydrogen bis(4-nitrophenoxide) dihydrate, 1, with deuterium replacing hydrogen in the bridge and the structural water molecules, has been determined crystallographically at 113, 173, and 295 K. The structure of 1 had previously been determined at similar temperatures (Kreevoy et al.). The O,O distances are 1.5-1.7 pm greater in the deuterated compound than in the undeuterated, at all three temperatures, providing another example of an Ubbelohde effect in a Speakman-Hadži compound. The temperature invariance of the Ubbelohde effect at temperatures up to room temperature is evidence against centralization of the hydron within this temperature range. It has previously been suggested (Kreevoy et al.) that simplification of the IR spectrum of 1 with increasing temperature is due to an increase in the rate of the hydron shift between the two basic oxygens. This suggestion is strengthened by the elimination of hydron centralization as an alternative. The O,O distance in 1 also increases with temperature, and the dihedral angle between the mean planes of the two aromatic rings decreases. Similarly, the increase in the O,O distance with isotopic substitution is accompanied by a small decrease in the dihedral angle; another geometric isotope effect. Ubbelohde effects in Speakman-Hadži compounds make the geometric isotope effects found computationally in the critical complexes for hydron, hydrogen atom, or hydride transfer more credible.